**5.2 PH**

158 Biogas

(Golueke, 2002). Most methanogens function in a pH range between 6.7 and 7.4, and optimally between 7.0 and 7.2. The greatest potential for a digester failure is a result of acid accumulation. This would occur if the amount of volatile solids loaded into the digester as fresh waste increased sharply. Maintaining pH is especially delicate in the start-up because fresh waste must undergo acid forming stages before any methane forming can begin, which will lower the pH. To raise the pH during the early stages, operators must add a

Several studies proved that the intensity of mixture in an CSTR digester has an effect on the process inhibition and the re-establishment of the organic overload (Hill & Bolte, 1989). Other researchers (Hill, 1990) studied the accumulation of acetate and propionate in a CSTR digester which treats municipal solid waste and the biosolides with an aggressive starting and an organic overload. They noted that while acetate was consumed thereafter, propionate persisted in the whole system and it started to decrease only after the reducing of mixture intensity. They also noted that a digester with a reduced mixture can tolerate a

The inhibiting compounds are one or the other present already in the substrate or product during degradation. The majority of the inhibitors are formed during the degradation of the substrate, such as VFA, LCVA, ammonia and sulphide. Some inhibitors are present already

The VFA is the main intermediate in anaerobic digestion, and it accumulates under the action of the non balance of the process. With low pH, the VFA becomes more toxic, due to

Ammonia comes mainly from the degradation of protein. A study on 18 central biogas stations in Denmark, proved that ammonia was significatif factor affecting the stability of the process (Hawkes & al., 1994). A concentration about 2 gN/l of ammonia will have no inhibiting effect on acetoclastic methanogens (Hill & Holmberg, 1988). However, the activity of methanogens is decreased during the increase in ammonia concentration, and total

The control of the anaerobic digesters is necessary to ensure a good operating of the digester. Since anaerobic digestion is a complex process implying several groups of microorganisms which are sensitive to several factors of operation, it is significant to be able to detect the non balance of the process at the beginning to take an action in time to prevent its failure. As with other biological processes, anaerobic digestion can be controlled by measuring the substrate convertion (COD or removed VS), the accumulation of intermediaries (VFA, pH, alkalinity, H2, CO), the formation of product (gas production rate,

buffer to the system, such as calcium carbonate or lime.

higher organic load than the digester with an intensive mixture.

in substrate, such as the heavy LCVA, and metals.

inhibition is reached for a concentration of 10 gN/l.

**5. Control parameters of the process of biogas** 

the increase of the non dissociated fraction.

**4.4 Intensity of mixture** 

**4.5 Composed toxic/inhibiting** 

CH4, CO2).

The pH is relatively easy to measure, and is often the only parameter of the liquid phase which is measured on line. The change of the pH can be an indicator, for the stability of anerobic digestion process. Since the micro-organisms can grow at only one specific pH range. The effluent pH can also affect the pH in the digester. The use of the pH as an indicator is normally based on the fact that a decrease of the pH corresponds to the accumulation of VFA. Some anaerobic systems apply the control of the pH where an acid or a base is added to ensure the suitable pH for the microbial growth.
